Since PTFE resins have a relatively high melting viscosity and are not dissolved by most solvents, fibers cannot be produced by a generally adopted method such as extrusion spinning of molten resins and resin solutions. Therefore, various specific manufacturing methods have been adopted conventionally. U.S. Pat. No. 2,772,444 proposes a method for manufacturing a PTFE fiber by emulsion spinning of a mixed solution of an aqueous dispersion solution of PTFE fine particles and viscose, followed by sintering of the PTFE at high temperatures to remove the viscose by thermal decomposition. However, the manufacturing cost of the PTFE by this method is high, whereas the strength of the fiber obtained is low, and therefore the strength of a product obtained by processing this fiber as a raw material also is low.
U.S. Pat. No. 3,953,566 and U.S. Pat. No. 4,187,390, for example, propose a method for manufacturing a high-strength PTFE fiber by slitting a PTFE film or sheet into a minute width, followed by stretching of the obtained tape. However, this method has a difficulty in maintaining a width of the tape obtained by slitting uniformly along the lengthwise direction. Also, there exists a problem that an end portion of the tape tends to be a fibril. For these reasons, there exists another problem that the fiber may break partially during the step of stretching the tape to a high degree.
U.S. Pat. No. 5,562,986 proposes a method for manufacturing cotton-like materials made of PTFE fibers having a branch structure by opening a uniaxially oriented article, specifically a uniaxially oriented film of a molded PTFE article by a mechanical force using a pin roll with a needle density of 20 to 100 needles/cm2. According to this method, however, the length of the obtained PTFE fibers mostly is not more than 150 mm, and it is difficult to obtain a PTFE filament.
WO96-00807 proposes a method for manufacturing cotton-like materials made of PTFE fibers having a branch structure by opening a uniaxially oriented film of a molded PTFE article with a mechanical force. According to this method, however, the density of the obtained PTFE fibers has a high specific gravity exceeding 2.15 g/cm3, thus making it difficult to obtain a light-weight final product.
In the case where the afore-mentioned PTFE oriented film is supplied to a revolving pin roll so as to produce a PTFE fiber, problems occur such as difficulty in making a single fibril thinner, nonuniform fineness and the occurrence of many losses from the end portions of the film supplied. Furthermore, a network structure of the filament is not uniform and therefore a branch structure of a branched PTFE short fiber obtained by cutting the filament also is not uniform and not stable.